Messages in this thread | | | From | Vincent MAILHOL <> | Date | Thu, 12 May 2022 08:23:58 +0900 | Subject | Re: [PATCH v2 2/2] x86/asm/bitops: __ffs,ffz: use __builtin_ctzl to evaluate constant expressions |
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On Thu. 12 May 2022 at 07:20, Nick Desaulniers <ndesaulniers@google.com> wrote: > On Wed, May 11, 2022 at 9:04 AM Vincent Mailhol > <mailhol.vincent@wanadoo.fr> wrote: > > > > __ffs(x) is equivalent to (unsigned long)__builtin_ctzl(x) and ffz(x) > > is equivalent to (unsigned long)__builtin_ctzl(~x). Because > > __builting_ctzl() returns an int, a cast to (unsigned long) is > > necessary to avoid potential warnings on implicit casts. > > > > For x86_64, the current __ffs() and ffz() implementations do not > > produce optimized code when called with a constant expression. On the > > contrary, the __builtin_ctzl() gets simplified into a single > > instruction. > > > > However, for non constant expressions, the __ffs() and ffz() asm > > versions of the kernel remains slightly better than the code produced > > by GCC (it produces a useless instruction to clear eax). > > > > This patch uses the __builtin_constant_p() to select between the > > kernel's __ffs()/ffz() and the __builtin_ctzl() depending on whether > > the argument is constant or not. > > > > ** Statistics ** > > > > On a allyesconfig, before applying this patch...: > > > > | $ objdump -d vmlinux.o | grep tzcnt | wc -l > > | 3607 > > > > ...and after: > > > > | $ objdump -d vmlinux.o | grep tzcnt | wc -l > > | 2600 > > > > So, roughly 27.9% of the call to either __ffs() or ffz() were using > > constant expression and were optimized out. > > > > (tests done on linux v5.18-rc5 x86_64 using GCC 11.2.1) > > > > Note: on x86_64, the asm bsf instruction produces tzcnt when used with > > the ret prefix (which is why we grep tzcnt instead of bsf in above > > benchmark). c.f. [1] > > > > [1] commit e26a44a2d618 ("x86: Use REP BSF unconditionally") > > http://lkml.kernel.org/r/5058741E020000780009C014@nat28.tlf.novell.com > > > > CC: Nick Desaulniers <ndesaulniers@google.com> > > Signed-off-by: Vincent Mailhol <mailhol.vincent@wanadoo.fr> > > Patch LGTM, though I find the location of the double unscores in the > names slightly against my taste. > > > --- > > arch/x86/include/asm/bitops.h | 38 ++++++++++++++++++++++------------- > > 1 file changed, 24 insertions(+), 14 deletions(-) > > > > diff --git a/arch/x86/include/asm/bitops.h b/arch/x86/include/asm/bitops.h > > index 6ed979547086..7cf5374ce403 100644 > > --- a/arch/x86/include/asm/bitops.h > > +++ b/arch/x86/include/asm/bitops.h > > @@ -224,13 +224,7 @@ static __always_inline bool variable_test_bit(long nr, volatile const unsigned l > > ? constant_test_bit((nr), (addr)) \ > > : variable_test_bit((nr), (addr))) > > > > -/** > > - * __ffs - find first set bit in word > > - * @word: The word to search > > - * > > - * Undefined if no bit exists, so code should check against 0 first. > > - */ > > -static __always_inline unsigned long __ffs(unsigned long word) > > +static __always_inline unsigned long __variable_ffs(unsigned long word) > > How about `variable___ffs`? Patch 1/2 used `variable_ffs` for `ffs`?
On a first glance, having the triple underscores in the middle of the name seemed odd. On second thought, it is more consistent with the rest, and I finally like the idea. Will be adopted in v3.
> > { > > asm("rep; bsf %1,%0" > > : "=r" (word) > > @@ -238,13 +232,18 @@ static __always_inline unsigned long __ffs(unsigned long word) > > return word; > > } > > > > -/** > > - * ffz - find first zero bit in word > > - * @word: The word to search > > - * > > - * Undefined if no zero exists, so code should check against ~0UL first. > > - */ > > -static __always_inline unsigned long ffz(unsigned long word) > > +/** > > + * __ffs - find first set bit in word > > + * @word: The word to search > > + * > > + * Undefined if no bit exists, so code should check against 0 first. > > + */ > > +#define __ffs(word) \ > > + (__builtin_constant_p(word) ? \ > > + (unsigned long)__builtin_ctzl(word) : \ > > + __variable_ffs(word)) > > + > > +static __always_inline unsigned long __variable_ffz(unsigned long word) > > `ffz` had no underscore. Regardless of `__ffs`, this should definitely > be `variable_ffz` IMO.
ACK.
> > { > > asm("rep; bsf %1,%0" > > : "=r" (word) > > @@ -252,6 +251,17 @@ static __always_inline unsigned long ffz(unsigned long word) > > return word; > > } > > > > +/** > > + * ffz - find first zero bit in word > > + * @word: The word to search > > + * > > + * Undefined if no zero exists, so code should check against ~0UL first. > > + */ > > +#define ffz(word) \ > > + (__builtin_constant_p(word) ? \ > > + (unsigned long)__builtin_ctzl(~word) : \ > > + __variable_ffz(word)) > > + > > /* > > * __fls: find last set bit in word > > * @word: The word to search > > -- > > 2.35.1 > > > > > -- > Thanks, > ~Nick Desaulniers
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